1. Field of the Invention
This invention relates generally to differential signal receivers with noise immunity and more particularly, it relates to an improved GTL input receiver which includes means for dynamically changing the current ratio of a pair of primary current steering devices so as to add hysteresis, thereby producing a higher margin of noise immunity.
2. Prior Art
As is generally well-known in the art of digital data communication systems, a number of data buses are typically used to transfer or communicate information among the various subsystems. Each of the data buses can be either of the unidirectional or bi-directional type. In the unidirectional type, a driver is located at one end of the data bus and a receiver is located at the other end of the data bus. In the bi-directional type, both a driver and a receiver (sometimes referred to as a "transceiver") are located at each end of the data bus. When information is transmitted over a data bus between two subsystems, it is extremely important to minimize the undesirable electrical transmission line reflection and/or noise generated when data is switched between a logic high ("1") state and a logic low ("0") state or vice-versa. In any event, it is always desirable to reduce such reflection and/or noise since it affects the net performance of the receiver.
Prior art attempts have been made heretofore so as to eliminate these factors, but they have required the necessity of redesigning of the back plane area of the digital data communication system. Further, the problems of reflections and/or noise were always unable to be completely eliminated and could only be reduced at best. As a result, the prior art arrangements still did not provide a receiver with a sufficient noise margin.
One such solution to this noise margin problem is provided in the prior art which utilizes a differential receiver in noisy environments so as to generate a high common mode rejection ratio. The common mode rejection eliminates the common noise factor which is superimposed on both inputs of the differential amplifier, but does not affect the induced noise at only an individual input. In order to overcome this deficiency, there is also known in the prior art of providing a hysteresis transfer function for the differential receiver. Once an output level has been established, small variations at the input to the receiver do not cause erroneous state changes at the output. Accordingly, the present output state will not switch to the alternate output state unless the input voltage changes to a value substantially different from the value that initiated its present state.
An example of a prior art differential receiver is a standard GTL (Gunning-Transistor-Logic) receiver 100 for receiving differential GTL signals V.sub.in1 and V.sub.in2 and for converting the same to a single-ended CMOS signal, as is illustrated in FIG. 1. The receiver 100 includes a reference current which is mirrored into two identical current steering transistor devices. The gates of the transistor devices are connected to input terminals for receiving the differential GTL signals. Typically, the differential GTL signals are analog signals in the range of 0.4 volts to 1.2 volts with a mid-point of 0.8 volts. Unfortunately, the input terminals of the receiver 100 are highly susceptible to noise. Thus, with a nominal input level of 0.8 volts, the noise margin of the receiver 100 is approximately 0.8 volts.+-.less than 50 millivolts. In other words, the input hysteresis is less than 50 mV which can cause the output to switch states when the difference between the differential input signals is less than 50 mV caused by the noisy environment.
Accordingly, it would be desirable to provide an improved GTL input receiver which has a higher immunity to noise so as to provide a better performance. The GTL input receiver of the present invention represents a substantial improvement over the prior art GTL receiver of FIG. 1. The present GTL receiver includes means for dynamically changing the current ratio of a pair of primary current steering devices so as to add hysteresis, thereby increasing the noise margin.